1. Field of the Invention
The present invention relates in general to an amplifier for amplifying a pair of signals indicating electrical potential of skin at two locations and more particularly to an electroencephalographic amplifier having a narrow band which is suitable for cortical evoked potential studies which use an input signal that matches the narrow band of the amplifier and to use with an evoked potential autorefractometry system.
2. Description of the Related Art
One type of electroencephalographic (EEG) study utilizes an input signal having a specific frequency which causes a cortical evoked potential that can be measured using EEG probes or electrodes on the scalp of the subject under study. Another type of EEG study relies on the prior identification of a specific frequency, such as alpha wave activity which increases immediately prior to a subject dozing off. Presently, such studies are conducted using an EEG amplifier which is capable of detecting a wide range of frequencies from direct current to several thousand hertz. In order to provide the capability to amplify signals with such a wide range of frequencies, conventional EEG amplifiers utilize chopper stabilization and cost ten to fifteen thousand dollars. However, despite the high cost and elaborate circuitry of such amplifiers, they do not eliminate all noise present in the environment in which they are used. This is particularly a problem in commerical environments when shielding from environmental electrical noise is normally not provided, such as in an optician's offie.
One example of such a conventional EEG amplifier is Model 611 manufactured by Beckman. This amplifier uses chopper stabilization circuitry and provides the flexibility described above. However, many types of EEG studies utilize two or more probes for sensing skin potential of the scalp in addition to a reference lead attached, for example, to an ear lobe. These types of studies display the difference between the potential of the two probes attached to the scalp. In order to perform such studies with conventional amplifiers, such as Beckman Model 611, one of the scalp probes is used as the reference lead and the other scalp probe is connected to the input to the amplifier. As a result, fluctuations in electrical potential sensed by the first scalp probe cause the internal ground of the input portion of the amplifier to fluctuate. As result, input impedance is limited to three to five kilohms.